Dripping sound in the drain pipe when gas furnace is on

I have a high efficiency gas furnace in the attic of 2 story house. The condensation is connected to the basement sewer ejection pump. Every time when the furnace is running, I can hear consistent dripping sound behind the wall somewhere between the first floor and basement every 5-10 seconds. It stopped immediately when furnace stops. I can see the water dripped out from the end of pipe to the sewage ejection pump in the basement, but where the sound comes from is about 15 ft away from the ejection pump. And the sound is more pronounced on the first floor. Since the basement is finished, I couldn't tell the plumbing behind the wall.

Where should I start to troubleshoot the problem? Thanks all. Happy Thanksgiving.

It might just be normal. PVC piping does transmit some sound, and can be annoying, depending on how it is run. From the attic to the basement, it has to fall a fair ways, and could gain a fair amount of velocity, then, if it hits an elbow, who knows. If the drain pipe isn't leaking, probably the only way to resolve this may be to relocate the pipe or add some insulation to help block the sound transmission. If it's a leak, you do need to find and fix it.